DDC and Parkinson disease: PD trials focusing on dopamine restoration strategies have targeted either aromatic L-amino acid decarboxylase (AADC) alone using AAV2 as vectors (AAV2-AADC) (Christine et al. 2019; Christine et al. 2009; Mittermeyer et al. 2012; Muramatsu et al. 2010) or a triad of key enzymes in the dopamine biosynthetic pathway including AADC, tyrosine hydroxylase (TH) and GTP-cyclohydrolase (GCH1) using lentivectors (LV-GCH1-TH-AADC; ProSavin→) (Palfi et al. 2014).